1. The hydrogenation of PAH cations: a journey guided by stability and magic numbers
Stéphanie Cazaux
Leon Boschman
Thomas Schlathölter
Ronnie Hoekstra
Geert Reitsma
Marco Spaans
Nathalie Rougeau
Sabine Morisset
Dominique Teillet Billy
From Clouds to Protoplanetary Disks: the Astrochemical Link
06th October 2015
VIDI fellow

2. PAHs in space
IR emission spectra in X (extra) galactic objects  vast array of Unidentified Infrared Emission (UIE)  Aromatic Infrared Bands (AIB)
AIB attributed to PAHs with sizes C
Leger & Puget 1984 A&A; Allamandola 1984 ASSL 108
Although the exact nature of carriers is unknown, The UIE bands are coomonly attributed to PAHs
Once they are electronically excited by the UV, PAHs relax to the groundstate mainly by emitting infrared light that comes from their C-C and C-H vibrations.
Peeters et al A&A 390
Tielens 2008 ARAA 46

3. PAHs in space
PAHs in space  Size? Weingartner & Draine 2001 ApJ, 548; Draine & Li 2007 ApJ 657
Neutral of cation? Allamandola 1999; Oomens et al ApJ 560
Aromatic or aliphatic? Li & Draine 2012 ApJL 760; Pilleri et al A&A 577
Hydrogenated or dehydrogenated? Bernstein et al ApJ 472; Montillaud et al A&A 552; Snow 1998 Nature
Composition of ISM
key physical conditions (UV, ionisation, dust column) Pilleri et al A&A 542, 69
Interstellar catalysts (formation of H2) Bauschlicher 1998 ApJL 509; Mennella et al ApJL 745; Thrower et al ApJ 752; Boschman et al. 2012, ApJL 761
 Study hydrogenation of PAHs  stability? H2 formation?
mid-infrared ( μm) spectrum can be used to trace key physical conditions along a given line of sight, such as the UV radiation field, the ionization parameter and the dust column density. These parameters are often difficult to determine independently from PDR models. The PAHTAT toolbox offers the opportunity to analyze mid-IR spectra using a limited number of parameters, that are associated with the physical properties of the dust and gas being observed.

4. Hydrogenation of coronene cations
Hydrogen source
Boschman, L., Reitsma, G., Cazaux, S., Schlathölter, T., Hoekstra, R.
Zernike Institute for Advanced materials in Groningen

5. Hydrogenation of coronene cations+1 +3 +5
Boschman, L. et al. 2012

6. Hydrogenation of coronene cations+1 +3 +5
Boschman, L. et al. 2012

7. +17H
+11H
+5H

8. Hydrogenation of coronene cations

9. Hydrogenation of coronene cations
Experiments show:
Hydrogenation increases with H exposure
Odd hydrogentation states are predominant
Occurrence of magic numbers of H attached:
Why are some hydrogenated states predominant?

10. First Hydrogenation 2.81 eV 2.14 eV 1.91 eV
DFT calculations  Equilibrium geometries, binding energies and transition states
Binding energy for 1st hydrogenation
2.81 eV
2.14 eV
1.91 eV
Outer edge
Inner Edge
Center

11. First Hydrogenations +1H +2H Outer edge carbon with Eb=2.81 eV
Radical + Radical  Little barrier of 0.01 eV  motion of the CH to form a CH2 group
+2H
Neighbor outer edge carbon with Eb=2.94 eV
Radical + close shell system  barrier of 0.07 eV  torsion of the C-C bond

12. Next Hydrogenations Alternance  dominance of odd states
H attachment to a radical even odd higher binding energy / small or no barrier
H attachment to a closed shell system odd  even lower binding energy / barrier
 dominance of odd states
Binding energies DO NOT alternate from odd to even
depend on reaction H + radical /closed shell
AND deformation of the system
Even
Odd

13. Next Hydrogenations
Outer edge
Inner Edge
Center 1 2

14. Next Hydrogenations 6 Outer edge Inner Edge Center 3 Barrier 0.1 eV 45 1 2

15. Next Hydrogenations 6 7 Outer edge Inner Edge Center 8 3
Barrier 0.1 eV 4 5 1 2 2 11 9 10

16. Next Hydrogenations 6 7 Outer edge Inner Edge Center 8 24 23 3 22 4
Barrier 0.1 eV 19 18 21 20 17 5 15 16 12 1 13 2 14 11 9 10

17. The sequence to hydrogenate coronene cations
Hydrogenation of coronene cations follow a definite sequence (from binding energies and attachment barriers) occurrence of stable states 5, 11 and 17 = Magic numbers
For these stable closed-shell cations: further hydrogenation requires appreciable structural changes high barriers
Barriers to add H  How do PAHs contribute to the formation of H2?
PDR model: H2 formation PAHs/dust with coronene as prototypical PAH in our model

18. H2 formation in PDRs H2 forms on PAHs for high Tgas and Tdust
Observations from PDRs can be explain by the formation of H2 on PAHs
PAHs are a high temperature pathway to molecular hydrogen
Boschman et al. 2015

19. Summary and conclusions
Experiments
Predominant hydrogenated states = magic numbers
Theory
Definite sequence to hydrogenate coronene cations  stable PAHH+ associated with the magic numbers found experimentally
Astrophysics
H2 can form efficiently on PAHs at high temperatures (Tgas > 200K)  explain PDRs
Remaining questions PAHH in space?
Parameter study to find PAHH in PDRs.
Resistance to UV?
 Determine their IR spectra  signatures.
Li & Draine 2012

20. Thank you

21. H2 formation in PDRs
H2 can form on PAHs through abstraction or photodesorption
For coronene as prototypical “PAHs”, photo processes are the most important route to form H2, and dominate the dust route in warm environments (Tgas > 200 K)
H2 on PAHs → necessary to reproduce the observations of PDRs.
Our model considers coronene while a distribution of sizes should be considered (do PAHs with < 40C survive?).
Larger sizes  H and H2 loss  as number of C 
Super-hydrogenated large PAHs (Montillaud 2013)  H abstraction
Study with distribution of sizes (including addition barriers) needed!

22. Dominique Teilley-Billy
Leon Boschman
Thomas Schlathölter
Ronnie Hoekstra
Geert Reitsma
Marco Spaans
Nathalie Rougeau
Sabine Morisset
Dominique Teilley-Billy
Thank you